Multi-drain semiconductor power device and edge-termination structure thereof

ABSTRACT

An embodiment of a semiconductor power device provided with: a structural body made of semiconductor material with a first conductivity, having an active area housing one or more elementary electronic components and an edge area delimiting externally the active area; and charge-balance structures, constituted by regions doped with a second conductivity opposite to the first conductivity, extending through the structural body both in the active area and in the edge area in order to create a substantial charge balance. The charge-balance structures are columnar walls extending in strips parallel to one another, without any mutual intersections, in the active area and in the edge area.

PRIORITY CLAIM

This application claims the priority benefit of U.S. patent applicationSer. No. 12/640,980, filed on Dec. 17, 2009, which application claimspriority to Italian Patent Application No. TO2008A000999, filed Dec. 29,2008, which applications are hereby incorporated by reference to themaximum extent allowable by law.

BACKGROUND

1. Technical Field

An embodiment of the present invention relates to a semiconductor powerdevice of a multi-drain type and to a corresponding edge-terminationstructure.

2. Discussion of the Related Art

In the last few years, a wide range of solutions have been developed forimproving the efficiency of semiconductor power devices, and inparticular for obtaining an increase of the breakdown voltage and adecrease of the output resistance.

For example, U.S. Pat. No. 6,228,719, U.S. Pat. No. 6,300,171, U.S. Pat.No. 6,404,010, and U.S. Pat. No. 6,586,798, which are incorporated byreference, describe vertical-conduction semiconductor power devices ofthe multi-drain (MD) type, wherein, within an epitaxial layer formingpart of a drain region having a given type of conductivity,charge-balance columnar structures are provided, having oppositeconductivity. These structures have a dopant concentration substantiallyequal and opposite to the dopant concentration of the epitaxial layer insuch a way as to provide a substantial charge balance. Charge balanceenables high breakdown voltages to be obtained, and moreover the highconcentration that the epitaxial layer can consequently assume enables alow output resistance (and reduced losses in conduction) to be achieved.

In order to manufacture the columnar structures, a sequence of steps ofgrowth of epitaxial layers of a first conductivity, for example of an Ntype, is envisaged, each step being followed by an implantation ofdopant of the second conductivity, in the example of a P type. Theimplanted regions are stacked and subjected to a subsequent process ofdiffusion of the dopant atoms so as to give rise to uniform columnarstructures.

Next, body regions of the power device are formed in contact with thecolumnar structures, in the active region, in such a way that thecolumnar structures constitute an extension of the body regions withinthe drain region.

The evolution of this technology has shown a progressive increase in thedensity of the elementary strips forming the devices in order toincrease further the charge concentration of the epitaxial layer andobtain devices that, given the same breakdown voltage (substantiallylinked to the height of the columnar structures), would present a loweroutput resistance. On the other hand, however, the increase in thedensity of the elementary strips has led to a reduction of the thermalbudget of the devices and a corresponding increase in the number ofsteps of epitaxial growth, and hence an increase in the manufacturingcosts and time and in the defectiveness intrinsically linked to theepitaxial growth.

Alternative techniques have hence been developed in order to obtain thecharge-balance structures, which envisage, for example, formation oftrenches within the epitaxial layer and subsequent filling of the sametrenches with semiconductor material appropriately doped to achieve thecharge balance. For example, in patent applications WO 2007/116420 andWO 2007/122646, which are incorporated by reference, techniques aredescribed for obtaining charge-balance structures in electronic powerdevices, which envisage the formation of trenches and filling thereofwithout any residual defectiveness via a particular technique ofnon-selective epitaxial growth performed in the same trenches.

t is also known that the formation of efficient edge-terminationstructures may be a key point for ensuring proper operation of the powerdevices. In fact, it is at the edge areas (i.e., the areas surroundingthe active area in which the electronic components are provided) thatthe highest number of breakdown phenomena occurs on account of thethickening of the electric field lines due to the presence ofdiscontinuities, such as sharp edges or the curvature of the dopedregions. Edge terminations have the function of reducing the intensityof the electric field locally so as to prevent peaks of intensity at theedges.

FIGS. 1-4 (which are not drawn to scale, as the subsequent figures arealso not to scale) show an example of an edge-termination structure of aknown type, for a vertical-conduction charge-balance power device. Inparticular, FIG. 1 shows a schematic and simplified top plan view,whilst FIGS. 2, 3 and 4 show cross sections taken along lines II-II,III-III and IV-IV indicated in FIG. 1, respectively.

The power device, designated by 1, is formed in a die 2 of semiconductormaterial, for example silicon. The die 2 has, in top plan view, agenerically rectangular or square shape; the borders and edges of thedie 2 correspond to the so-called “scribe lines” (designated by LT), atwhich the starting wafer of semiconductor material has been cut. In thedie 2 it is possible to define a peripheral portion 2 a, adjacent to thescribe lines, and a central portion 2 b, in which the power device 1 isphysically provided.

The die 2 comprises a substrate 3 having a first type of conductivity,for example of an N⁺⁺ type, and an epitaxial layer 4, formed on thesubstrate 3, also having the first type of conductivity, in the exampleof an N type. Within the epitaxial layer 4 it is possible to distinguishan active area 4 a, designed to house elementary electronic components50 (in the example, MOS transistors) of the power device 1, and an edgearea 4 b, designed to house an edge-termination structure of the deviceand adjoining the peripheral portion 2 a of the die 2. In particular,the epitaxial layer 4 constitutes a common drain surface region for theplurality of elementary electronic components 50 (the MOS transistors)forming the power device 1.

The edge-termination structure comprises a ring region 5, in particulara region doped with a second type of conductivity, of a P type, with lowconcentration, for example, lower than 10¹⁶ at/cm³, formed in a surfaceportion of the epitaxial layer 4. The ring region 5 is provided withinthe edge area 4 b, surrounds the active area 4 a completely (forming aring around it), and has an area of superposition with a peripheralportion of the same active area 4 a. In particular, the ring region 5has a rounded-off and curved profile in such a way as to reduce localconcentrations of the field lines.

Charge-balance structures 7 (which have in cross section a columnconformation, see in particular FIGS. 2-4) traverse the epitaxial layer4 substantially throughout its thickness, stopping at a certain distancefrom the substrate 3, both at the active area 4 a and at the ring region5 in the edge area 4 b. The charge-balance structures 7 are, forexample, obtained through successive steps of epitaxial growth andimplantation of dopant atoms in order to obtain stacked doped regions,and through a final step of diffusion of the dopant atoms.

The charge-balance structures 7 follow the layout of the regions inwhich they are formed, and are constituted by doped regions having thesecond type of conductivity (P) and a doping level such as to create asubstantial charge balance. In particular, in the active area 4 a thecharge-balance structures 7 are constituted (in plan view, see FIG. 1)by first strips 7 a, having a substantially rectilinear extension,parallel to one another and to a first side of the die 2 (and to a firstaxis x), which repeat periodically and at substantially the samedistance in a direction parallel to a second side of the die 2 (and to asecond axis y, orthogonal to the first axis x). Instead, in the edgearea 4 b the charge-balance structures 7 follow the pattern and theprofile of the ring region 5, within which they are housed, and are madeup of second strips 7 b, once again parallel to and set at substantiallythe same distance from one another, each of which is constituted by: afirst rectilinear portion parallel to the first side of the die 2 (andto the first axis x); a second rectilinear portion parallel to thesecond side of the die 2 (and to the second axis y); and a curvedconnecting portion between the first rectilinear portion and the secondrectilinear portion (in particular having substantially the same radiusof curvature as the ring region 5).

In particular, given their columnar extension in the thickness of theepitaxial layer 4, the charge-balance structures 7 constitute verticalwalls or diaphragms extending in strips within the same epitaxial layer4. In addition, in current design rules, the number of the second strips7 b that occupy the ring region 5 is determined by the dimension of thering and by the pitch (in terms of spacing and size) of the first strips7 a in the active area 4 a.

Body wells 9 are present within the active area 4 a, having the secondtype of conductivity (P) and contacting each first strip 7 a of thecharge-balance structures 7, at the surface portion of the epitaxiallayer 4. In particular, the first strips 7 a constitute extensions ofthe body wells 9 within the drain region in the epitaxial layer 4.Source regions 10, having the first type of conductivity (N), areprovided inside each body well 9. In particular, in the area ofsuperposition between the active area 4 a and the ring region 5, theoutermost body wells 9 join the same ring region 5. In addition, in theedge area 4 b, the second strips 7 b of the charge-balance structures 7are joined to one another by the ring region 5.

The power device 1 further comprises, on the surface of the epitaxiallayer 4, a first dielectric region (for example, made of silicon oxide)12, having a greater thickness at the edge area 4 b and a smallerthickness in the active area 4 a, where it provides the gate-oxideregions of the elementary electronic components 50. A gate region (madeof polysilicon or other conductive material) 14 is provided on the firstdielectric region 12; on the gate-oxide regions, the gate region 14provides the gate structures of the elementary electronic components 50.

In addition, a second dielectric region (for example, made of fieldoxide) 15 covers the first dielectric region 12 and the gate region 14.The second dielectric region 15 is traversed, at the edge area 4 b, by agate metal contact 18, designed to contact the gate region 14. Inaddition, the second dielectric region 15, the first dielectric region12, and the gate region 14 are traversed, in the active area 4 a, by asource metal contact 16, extending to contact and short-circuit thesource regions 10 and the body wells 9. At the periphery of the edgearea 4 b (adjacent to the peripheral portion 2 a of the die 2), thesurface of the epitaxial layer 4 is left exposed so as to enable anequipotential-ring (EQR) metal contact 19 to contact a doped region 20,in particular a doped region having the first type of conductivity (N),provided in the surface portion of the epitaxial layer 4. The dopedregion 20 is set at a distance from the ring region 5, and has the samering layout as the latter, surrounding it completely. The contact region20 has the function of bringing to the surface the drain potential so asto limit horizontally the electric field lines in reverse biasing.

In analysing the cross sections of FIGS. 2-4, it is to be noted inparticular that the cross section of FIG. 2 is taken in a directiontransverse to the direction of extension of the charge-balancestructures 7, and that the cross sections of FIGS. 2 and 3 are bothtaken along the direction of extension of the charge-balance structures7, but on the outside and on the inside, respectively, of a first strip7 a.

It has been shown that power devices of the type described, althoughhaving considerable advantages as compared to traditional solutions, maybe subject to phenomena of early breakdown that can jeopardize theirperformance or, in the worst case, prevent their subsequent use (i.e.,destroy them).

SUMMARY

An embodiment of the present invention overcomes the above drawbacks andfurther improves a charge-balance power device.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, one or moreembodiments are now described, purely by way of non-limiting example andwith reference to the attached drawings, wherein:

FIG. 1 is a schematic and simplified top plan view of a portion of apower device of a known type, in particular corresponding to anedge-termination structure;

FIGS. 2, 3, and 4 show cross sections of the device of FIG. 1, takenalong lines II-II, III-III, and IV-IV of FIG. 1, respectively;

FIG. 5 shows an IN plot representing the reverse breakdowncharacteristic of the device of FIG. 1;

FIG. 6 shows a microscope photograph of the result of anemission-microscopy (EMMI) analysis on the portion of the power deviceof FIG. 1;

FIG. 7 a is a schematic and simplified top plan view of a power deviceaccording to an embodiment of the present invention;

FIG. 7 b is a top plan view of a portion of the device of FIG. 7 a, inparticular regarding an edge-termination structure thereof;

FIGS. 8, 9 and 10 show cross sections of the device of FIGS. 7 a and 7b, taken along lines VIII-VIII, IX-IX, and X-X of FIG. 7 b,respectively;

FIGS. 11-12 show plots representing the breakdown characteristics of thedevice of FIGS. 7 a and 7 b;

FIG. 13 shows a microscope photograph of the result of an EMMI analysison the portion of the power device of FIG. 7 b;

FIGS. 14 and 15 show top plan views similar to that of FIG. 7 b,regarding further embodiments of the present invention;

FIG. 16 shows a cross section of the power device shown in FIG. 15,taken along the line XVI-XVI; and

FIGS. 17 and 18 show top plan views similar to that of FIG. 7 b,regarding further variant embodiments of the present invention.

DETAILED DESCRIPTION

It has been discovered and verified experimentally that the criticalfeatures of early breakdown afflicting the multi-drain power devices ofthe type previously described derive principally from the presence ofdiscontinuities in the charge balance occurring within the edge area 4b, and specifically in the ring region 5.

In particular (see again FIG. 1), these discontinuities occur at thejoining points between the charge-balance structures 7 in the activearea 4 a and those in the edge area 4 b, and more particularly in thecontact points between the first strips 7 a and the second, innermost,strip 7 b (i.e., the one closest to the active area 4 a), at which thecharge may not be balanced and an excess of charge occurs. In detail,two types of intersection zones with marked charge unbalancing aregenerated: areas that in top plan view can be defined as “cuspidal”,designated by P_(C) in FIG. 1, at the intersection between a first setof first strips 7 a and the curved portion of the aforesaid second,innermost, strip 7 b; and areas that in top plan view are T-shaped,designated by P_(T) in FIG. 1, at the intersection between a second setof first strips 7 a and the second rectilinear portion of the second,innermost, strip 7 b.

These areas of excess of charge are a source of early triggering ofbreakdown having a triangular reverse-biasing characteristic, as shownin FIG. 5, whatever the number of the second strips 7 b that are presentin the ring region 5.

The early triggering phenomena are moreover highlighted by the EMMIanalysis shown in FIG. 6, regarding the same portion of device of FIG.1, in which evident light emission phenomena are visible at the areaswith higher charge unbalancing (cuspidal areas P_(C)).

Furthermore, it is noted (reference may be made to the cross sections ofFIGS. 2-4) that the sections of the edge area 4 b, in a directionorthogonal or parallel to the direction of extension of the first strips7 a, are different from one another, due to the asymmetry introduced inthe structure by the arrangement of the second strips 7 b in the ringregion 5, in particular in terms of the local charge balance that it ispossible to obtain and of the number of charge-balance structures 7joined to the ring region 5.

In order to overcome these critical features, an embodiment of thepresent invention envisages a different configuration of thecharge-balance structures, such that the charge-balance structuresoccupy substantially uniformly and without any substantial discontinuitythe entire area of the power device.

In detail, and as is shown in FIGS. 7 a and 7 b, where the samereference numbers are used for representing elements similar to othersdescribed previously, the charge-balance structures, here designated by7′, of the power device, here designated by 1′, comprise stripsextending parallel to one another over the entire area of the die 2, andin particular throughout the active area 4 a (first strips 30 a) andthroughout the edge area 4 b (second strips 30 b), without any mutualintersection. In addition, the strips 30 a, 30 b extend also to theperipheral portion 2 a of the die. The strips 30 a, 30 b extend, forexample, parallel to the first side of the die 2 and to the first axisx, repeating periodically along the second axis y, at a substantiallyuniform distance of separation. In particular, the charge-balancestructures 7′ do not follow in this case the layout of the ring region5. It is to be noted that the first strips 30 a extend from the activearea 4 a into the edge area 4 b, and also that the first, outermost,strip 30 a (i.e., the one closest to the edge area 4 b) is parallel tothe adjacent second, innermost, strip 30 b (i.e., the one closest to theactive area 4 a), without intersecting it throughout its extension.

This arrangement makes it possible to avoid discontinuity areas in thecharge balance (in particular the cuspidal areas within the ring region5), that have been shown to be at the origin of the points of localcharge unbalancing in the epitaxial layer 4. In particular, the factthat all the charge-balance structures 7′ extend in a parallel way meansthat high electric field is always sustained by parallel “walls” ofopposite charge, without intersecting structures that may locally inducean increase in the electric field.

The sections of the edge area 4 b, in a direction orthogonal or parallelto the direction of extension of the strips 30 a, 30 b, as shown inFIGS. 8-10, are in this case clearly equivalent in terms of the localcharge balance that it is possible to obtain given that all the stripsof the charge-balance structures 7′ extend parallel to one another, andin particular demonstrate how charge balance is obtained in the edgearea 4 b in a way substantially similar to the active area 4 a.

It has been shown experimentally that the structure described, with theconsequent elimination of local singularities, makes it possible toinhibit triggering of early breakdown. In this regard, FIG. 11 shows theresulting IN characteristic of the power device 1′, whilst FIG. 12 showsthe comparison between the evolutions of the normalized breakdownvoltage BVdss (breakdown voltage between drain and source with the gateset at ground potential) as a function of the net charge Φ_(n) (chargeof a P type minus charge of an N type) given in arbitrary units (a.u.),in the power device 1′ according to an embodiment of the presentinvention (solid line) and in a traditional device (dashed line). Theshift observed experimentally of the peak of BVdss (that should ideallycorrespond to the charge balance, Φ_(n)=0), indicates that the localunbalancing in the structure may induce clamping of the maximumbreakdown value.

An optimization of the layout in the edge area 4 b may enableelimination of this structural clamping in the breakdown voltage.

Also the EMMI analysis in breakdown conditions (FIG. 13) confirms theprevious results, highlighting the presence of a substantially uniformemission between the strips of the charge-balance structures 7′.

As is shown in FIG. 14, a different embodiment of the present inventionenvisages that the strips 30 a, 30 b of the charge-balance structures 7′do not extend throughout the entire surface of the die 2, but stopinside the edge area 4 b, before reaching the peripheral portion 2 a ofthe die 2. For example, the strips 30 a, 30 b interrupt beyond the ringregion 5, in an intermediate point between the ring region 5 itself andthe EQR metal contact 19 and the corresponding doped region 20. Inparticular, beyond the intermediate point there is no presence either ofprolongations of the first strips 30 a or prolongations of the secondstrips 30 b, or of the same second strips 30 b.

A further embodiment of the present invention, shown in FIG. 15 (in topplan view) and in FIG. 16 (in a section transverse to the direction ofextension of the strips 30 a, 30 b), further envisages the possibilitythat the edge-termination structure of the power device 1′ does notcomprise the ring region 5 in the edge area 4 b. In the example shown,the strips 30 a, 30 b extend again parallel to one another and in auniform way, over the entire area of the die 2, in the active area 4 aand in the edge area 4 b of the epitaxial layer 4. This embodiment maysolve the problem of “charge unbalancing” induced by the ring region 5,in the cases where it is possible to verify that the accumulation ofcharges due to the presence of points with high electric field (whichare no longer eliminated by the ring region) does not lead to asignificant decrease in reliability. In fact, the ring region 5introduces a surface charge at the edge area 4 b, which adds (with itssign) to the charge present in the charge-balance structures,consequently introducing in the proximity of the surface a certain locallack of uniformity in the charge balance.

Advantages of the semiconductor power device and of the correspondingedge-termination provided according to one or more embodiments of thepresent invention are clear from the foregoing description.

In particular, an embodiment removes, in multi-drain power devicesbasing their operation on charge balance in the epitaxial layeroperating as extension of the drain, any discontinuity in the chargebalance and consequently early breakdown phenomena, rendering thebreakdown characteristic hard. The periodic and uniform structure of thecharge-balance structures is maintained also in the edge region of thepower device, providing a substantial charge balance in the drain regionnot only in the active area but also at the edge, and preventing theedge-termination structure from introducing significant singularitiesand significant local excesses of charge. In particular, the improvementof the performance in reverse biasing enables improvement of theindustrialization of the process and the quality and reliability of thefinal devices produced.

Thanks to charge balancing, it is also possible to obtain values ofresistivity of the epitaxial layer lower than 2 Ω·cm, and values ofinhibition voltage comprised between 100 and 1500 V, as the thickness ofthe epitaxial layer varies.

Finally, it is clear that modifications and variations may be made towhat is described and illustrated herein, without thereby departing fromthe scope of the present disclosure.

In particular, as is shown in FIG. 17 in an embodiment, a first spacingd₁ may be provided in the active area 4 a between the first strips 30 a,and a second spacing d₂, different from the first, between the secondstrips 30 b in the edge area 4 b. In the example shown, the distancebetween the first strips 30 a is greater than the distance between thesecond strips 30 b (but also the opposite solution may be adopted,d₂>d₁).

As is shown in FIG. 18, in an embodiment the strips 30 a, 30 b may havea different size (in particular, a different width L in a directiontransverse to their main direction of extension) in the active area 4 awith respect to the edge area 4 b. For example, the first strips 30 amay have a transverse dimension L₁, and the second strips 30 b a secondtransverse dimension L₂, smaller than the first one (once again, theopposite solution may be alternatively adopted, L₂>L₁).

In particular, the aforesaid alternative embodiments may make itpossible to afford greater margins of freedom in redefining the “designrules” in order to offset the charge unbalancing induced by the ringregion 5 (in the case where it is not possible or desirable to eliminateit in order not to risk reliability problems), optimizing the size andspacing of the strips of the charge-balance structures in the edge area4 b.

Furthermore, in an embodiment, the charge-balance structures 7′, insteadof extending only within the epitaxial layer 4, may also reach thesubstrate 3 and terminate within the same substrate 3.

Clearly, different techniques may be used for obtaining thecharge-balance columnar structures. For example, as described in patentapplication No. WO 2007/006503, which is incorporated by reference, thesuccessive steps of epitaxial growth and implantation for creatingsuperimposed implanted regions, instead of being designed to form thecolumnar structures, may be designed to form regions which are todefine, between one another, the charge-balance columnar structures.Alternatively, as described in patent application No. EP-A1-1911075,which is incorporated by reference, it is possible to envisagesuccessive superimposed implantations (in corresponding regions ofepitaxial growth) to provide both the charge-balance columnar structuresand the regions of opposite conductivity set between the same columnarstructures. Alternatively, it may be possible to use the non-selectivetechnique of epitaxial growth within trenches, substantially asdescribed in the aforesaid patent applications Nos. WO 2007/116420 andWO 2007/122646, which are incorporated by reference.

One or more of the embodiments described may also be used, with theappropriate small modifications, to obtain a generic charge-balancepower device (for example, a bipolar diode, a Schottky diode, a BJT, anIGBT, etc.) and the corresponding edge-termination structure. Inparticular, it is clear that, in the case, for example, of a diode powerdevice, the epitaxial region constitutes an extension of a cathodeterminal (even though usually also this power device is identified bythe term “multi-drain”). Furthermore, it is evident that it is possibleto obtain dual structures in which the charge balance is provided bymeans of formation of columnar structures with N doping in an epitaxiallayer with P doping.

From the foregoing it will be appreciated that, although specificembodiments have been described herein for purposes of illustration,various modifications may be made without deviating from the spirit andscope of the disclosure. Furthermore, where an alternative is disclosedfor a particular embodiment, this alternative may also apply to otherembodiments even if not specifically stated.

1. A method, comprising: forming an active region in a layer of a firstconductivity type adjacent to a corner region of the layer; and formingin the corner region of the layer at least one first column having asecond conductivity type and being substantially straight.
 2. The methodof claim 1, wherein: the first conductivity type comprises N type; andthe second conductivity type comprises P type.
 3. The method of claim 1,further comprising forming in the corner region of the layer at leasttwo substantially parallel first columns having a second conductivitytype and substantially a same length.
 4. The method of claim 1, furthercomprising forming in the corner region of the layer at least twosubstantially parallel first columns having a second conductivity typeand different lengths.
 5. The method of claim 1, further comprisinggrowing the layer epitaxially over a substrate.
 6. The method of claim1, further comprising forming in the active region at least one secondcolumn having the second conductivity type and substantially parallel tothe at least one first column.
 7. The method of claim 1, furthercomprising forming in the active region at least one second columnhaving the second conductivity type, substantially parallel to the atleast one first column, and having a length different from a length ofthe at least one first column.
 8. The method of claim 1, furthercomprising forming in the active region at least one second columnhaving the second conductivity type, substantially parallel to the atleast one first column, and having a length substantially the same as alength of the at least one first column.
 9. The method of claim 1,further comprising: forming in the corner region at least twosubstantially parallel first columns having a second conductivity typeand each substantially having a first width; forming in the activeregion at least two second columns having the second conductivity type,substantially parallel to the at least two first columns, and eachsubstantially having a second width that is different from the firstwidth.
 10. The method of claim 1, further comprising: forming in thecorner region at least two substantially parallel first columns having asecond conductivity type and each separated from an adjacent firstcolumn by substantially a first distance; and forming in the activeregion at least two second columns having the second conductivity type,substantially parallel to the at least two first columns, and eachseparated from an adjacent second column by substantially a seconddistance that is different from the first distance.
 11. The method ofclaim 1, further comprising forming in a surface portion of the cornerregion and at least partially around the active region a boundary regionhaving the second conductivity type.
 12. The method of claim 1, furthercomprising forming in a surface portion of the corner region over atleast one of the first columns and at least partially around the activeregion a boundary region having the second conductivity type.
 13. Themethod of claim 1, further comprising forming in a surface portion ofthe corner region integral with at least one of the first columns and atleast partially around the active region a boundary region having thesecond conductivity type.
 14. The method of claim 1, further comprisingforming in the corner region adjacent to a surface of the layer a biasregion having the first conductivity type at least partially around theactive region.
 15. The method of claim 1, wherein forming the at leastone first column comprises: forming a substantially straight trench inthe layer; and forming in the trench a material having the secondconductivity type.
 16. The method of claim 1, wherein forming the atleast one first column comprises: forming a substantially straighttrench in a first portion of the layer; forming in the trench a materialhaving the second conductivity type; and forming a second portion of thelayer over the trench.
 17. The method of claim 1, further comprising:forming in the corner region a peripheral region that borders the activearea; forming at least a portion of a doped equipotential ring of thefirst conductivity type that borders the peripheral region; and forminga lightly doped semiconductor region of the second conductivity typealong a surface of the corner region that extends from a periphery ofthe active area throughout a majority of the peripheral region andterminates a substantially uniform distance from the doped equipotentialring.
 18. The method of claim 1, wherein one or more of the at least onefirst columns extend straight into the active region and furthercomprising forming gate structures for transistors adjacent surfaceregions of the one or more first columns in the active region.
 19. Themethod of claim 18, further comprising forming source regions for twotransistors over the one or more first columns in the active region. 20.A method, comprising: forming an active region of a power device in alayer of a first conductivity type adjacent to a corner region of thepower device; forming in the corner region plural columnar structureshaving a second conductivity type and being substantially straight; andforming a contact ring of the first conductivity type surrounding theactive region and running through the corner region.
 21. The method ofclaim 20, further comprising forming one or more of the plural columnarstructures to be substantially parallel and to extend across the activeregion.
 22. The method of claim 21, wherein the one or more columnarstructures in the active region are substantially parallel to the pluralcolumnar structures in the corner region.
 23. The method of claim 20,further comprising forming in a surface portion of the corner region andextending substantially entirely around the active region a boundaryring region having the second conductivity type.
 24. The method of claim23, wherein the boundary ring region is formed over one or more of theplural columnar structures.
 25. The method of claim 23, wherein theboundary ring region connects one or more of the plural columnarstructures.
 26. The method of claim 23, wherein the plural columnarstructures terminate at approximately an outer edge of the boundary ringregion.
 27. The method of claim 23, wherein the plural columnarstructures terminate between an outer edge of the boundary ring regionand the contact ring.
 28. The method of claim 20, wherein the pluralcolumnar structures terminate at approximately a uniform distance fromthe contact ring.
 29. The method of claim 20, further comprising:forming columnar structures having the second conductivity type in theactive region; and forming gate structures for transistors adjacentsurface regions of the columnar structures in the active region.
 30. Themethod of claim 20, further comprising: forming source regions for twotransistors over each of at least two plural adjacent columnarstructures of the second conductivity type within the active region; andforming two gates adjacent each of the source regions within the activeregion.